EP3326285B1 - Apparatus for avoiding harmful bearing currents - Google Patents

Description

The present invention relates to a device for reducing and in particular for avoiding harmful bearing currents in an electrical machine, such as an EC motor, and an electrical machine which is equipped with such a device.

Today's variable-speed motors are mainly fed by voltage intermediate circuit converters. However, the supply by the voltage intermediate circuit converter leads to bearing currents in the motor bearings.

Such a current flow through the bearings can lead to damage or even complete failure in electrical machines with roller and plain bearings. So-called shaft voltages can occur during mains operation, which are induced in a conductor loop consisting of the shaft, the two bearings, the end shields and the housing.

The background and physical causes of shaft voltages are extensively described in the literature. In simple terms, due to asymmetries in the magnetic circuit that do not add up to "zero", magnetic fluxes occur within the electrical machine, which cause said magnetic ring flux.

To generate the required current form, an inverter circuit (inverter) is used which, on average, applies a voltage to the terminals of the EC motor using PWM, so that the desired current form is set. The inverter typically works with a switching frequency outside the audible range (> 16kHz).

When the electrical machine is supplied from a pulse-controlled inverter, a second type of electrical bearing stress occurs, namely a capacitively coupled-in bearing voltage. The switched pulse pattern of the inverter results in a common mode voltage (common mode voltage) with respect to earth at the switching frequency of the inverter. This common mode voltage is also transferred to the warehouse via capacitive coupling networks. The physical cause of this type of bearing voltage are the voltage changes at the inverter output, which are generated by the fast switching processes, and the control process of the pulse-controlled inverter, which is directly related to the common mode voltage.

This creates a tension between the inner and outer ring of the bearing and the ball of the bearing running on an insulating oil film, the oil film between the inner and outer ring of the bearing can be considered electrically as a capacitance. If the insulation breaks down due to insufficient insulation of the oil film or high bearing voltages, the oil film capacity discharges and the charge carriers between the inner and outer bearing rings are balanced (electric discharge machining).

Depending on the design of the motor, such bearing currents can lead to premature failure of the bearings, which leads to the formation of ripples on the bearing treads and to decomposition of the bearing grease.

To remedy this, current-insulated bearings, e.g. B. bearings with ceramic insulation on the outer ring or hybrid bearings with ceramic rolling elements used. However, since these bearings are very expensive, such a solution is not ideal for mass production.

Further remedial measures are known from the prior art. This is how the publications teach EP 1 445 850 A1 respectively. DE 10 2004 016 738 B3 to use a device for protecting a bearing of an electric machine, which provides a compensation circuit or a compensation device for generating a compensation current for the compensation of an interference current through the bearings.

An alternative solution is achieved by a targeted short circuit between the bearing rings. The US 20080088187A1 teaches e.g. B. to provide a conductive connection between the rotor and stator and to realize a short circuit between the rotor and stator via a spring. Alternatives to this are also known, in which a bypass element (for example a bypass capacitor) is used between the bearings.

It is disadvantageous, among other things, that the stator and rotor are conductively connected to one another. Thus, the isolation required between the Windings and the stator are reinforced accordingly. Signs of wear also appear on the sliding contact and an additional bypass element also causes additional costs.

Another known measure consists in isolating the shaft and overmolding the stator. Due to the additional insulation layer between the shaft and the stator, part of the bearing voltage is reduced over the insulation. The disadvantage here is that the bearing stresses that occur can nevertheless be sufficiently high that undesirable bearing currents occur.

A solution, such as the impedance matching of the end shield, such as. B. from the US 2011/0234026 A1 known, is only suitable for designs in which conductive end shields are provided at all.

From the publication of S. Schroth, D. Bortis, JW Kolar "Impact of Stator Grounding in Low Power Single-Phase ECMotors, Proceedings of the 29th Applied Power Electronics Conference and Exposition (APEC 2014), Texas, Houston, USA, March 16-20, 2014 " A solution is known in which the stator is constructed in an isolated manner and a capacitive connection of the stator to the earth potential takes place. The problem here is that the connection of the stator to the earth potential has an influence on the filter properties of the EMC filter and has a significant effect The EMC properties deteriorate because the impedance to the earth is reduced at the same time, which results in a significantly increased filter volume, which means that the motor has to be adapted individually so that such a solution is also not universally applicable .

The invention is therefore based on the object of providing a solution with which the undesired bearing currents can be effectively reduced or completely prevented, in which the aforementioned disadvantages do not occur and which can be used universally.

This object is achieved by means of a device for avoiding harmful bearing currents in an electrical machine with the features of claim 1, and with a motor with the features of claim 10.

The basic idea of the invention is to be seen in the fact that the protection of the bearing against undesired bearing currents is achieved by connecting the stator to a high-frequency potential which is stable with respect to the earth potential, preferably the electronic potential, the connection either directly as a short-circuit connection or alternatively via a capacitance or an impedance takes place, which at the same time ensures the insulation gap required for protection against accidental contact by the impedance introduced.

The electrical connection of the stator to a stable electronics potential via any impedance can therefore be realized ohmic, inductive, capacitive or by integrating entire network topologies.

In order to enable variable-speed operation of a motor, the motors are fed via inverters with a correspondingly high clock frequency. A typical converter with a DC link (also known as a U-converter) regularly consists of a rectifier, the DC link with constant voltage and an inverter, whereby a three-phase voltage system is generated from the constant DC link voltage. The DC link voltage is related to the earth potential via the rectifier and / or the EMC filter. As a result of the pulsation, a voltage occurs between the earth potential and the motor lines that corresponds to half the DC link voltage (± U _ZK / 2). This voltage occurs in all three strands as an in-phase common mode voltage.

According to the invention, the aforementioned stable potential of the electronics to which the stator is electrically connected is a potential of a preferably filtered U-converter that is different from the ground potential. So can be the connection either with the DC link potential DC + of the DC link or the ground potential of the electronics or DC-, the potential before or after the rectifier of the rectifier circuit, the potential within the EMC filter or alternatively also the output potential of the low voltage supply or the switching power supply.

According to the invention a device for reducing and / or avoiding harmful bearing currents in an electrical machine is proposed, such as. B. a three-phase EC motor, with a rotor and with an insulated stator, wherein a bearing outer ring and a bearing inner ring are provided between the rotor and stator, further comprising a connection electronics for connecting the motor, the stator by an electrical connection with a high frequency is connected to a potential tap of the connection electronics with respect to the earth potential which is different and to this stable electronics potential.

In a preferred embodiment of the invention it is provided that the electrical connection between the stator and the electronic potential is formed via an intermediate capacitance or via any intermediate impedance.

Alternatively, it can be provided that the electrical connection between the stator and the electronic potential is designed as an electrical short-circuit connection. The stator is thus directly at the electronic potential during operation of the motor.

• dem Potential des Zwischenkreises, vorzugsweise dem positive Zwischenkreispotential (DC+)
• dem Massepotential des Zwischenkreises (15),
• dem Potential am Mittelabgriff (15') des Zwischenkreises (15) oder
• dem negativen Zwischenkreispotential (DC-).

In a further advantageous embodiment of the device according to the invention, it is provided that the connection electronics comprise at least one rectifier, an intermediate circuit and an inverter and the potential tap for connecting the stator is provided or arranged in the connection electronics such that the electronic potential connected to the stator is one of the im The following potentials correspond to:

• the potential of the intermediate circuit, preferably the positive intermediate circuit potential (DC +)
• the ground potential of the intermediate circuit (15),
• the potential at the center tap (15 ') of the intermediate circuit (15) or
• the negative intermediate circuit potential (DC-).

Alternatively, the potential tap for connecting the stator to the electronic potential can also be a tap immediately before or immediately after the rectifier. In this case, it can be provided in a further development of the invention that a network which reduces impedance in the frequency range of the bearing voltage is connected in parallel with one, several or all diodes of the rectifier.

In a further alternative embodiment of the invention, the connection electronics furthermore have an EMC filter on the input side and the potential tap for the stator is a tap within the EMC filter.

Further alternatively, it can be provided that if the connection electronics are connected to a low-voltage supply or a switching power supply, the potential tap for the stator is provided as a tap at the outputs of the low-voltage supply or the switching power supply.

Another aspect of the present invention relates to an electric motor, preferably an EC motor, which is designed with a device as described above.

Other advantageous developments of the invention are characterized in the subclaims or are shown below together with the description of the preferred embodiment of the invention with reference to the figures.

Fig. 1

eine Darstellung des Gesamtsystems bei einer Anbindung des Stators an das Elektronikpotential,

Fig. 2

eine Modellierung des Kapazitätsnetzwerkes,

Fig. 3

ein Ersatzschaltbild der parasitären Lagerkapazitäten und dem Hauptstrompfad des Lagerstromes,

Fig. 4

ein Ersatzschaltbild der parasitären Lagerkapazitäten bei der Anbindung des Stators mit dem zufolge umgeleiteten Erdstrompfad,

Fig. 5

ein schematisch vereinfachtes Ersatzschaltbild zur Bestimmung der Lagerspannung gemäß der Ausführung nach Figur 3,

Fig. 6

vergleichende Messkurven der Gleichtaktspannung und Lagerspannung (ohne und mit Anbindung des Stators),

Fig. 7

eine alternative Ausführungsform einer erfindungsgemäßen Vorrichtung,

Fig. 8

eine weitere alternative Ausführungsform einer erfindungsgemäßen Vorrichtung in einer zweiten Variante,

Fig. 9

eine zweite alternative Ausführungsform einer erfindungsgemäßen Vorrichtung in der zweiten Variante,

Fig. 10

eine dritte alternative Ausführungsform einer erfindungsgemäßen Vorrichtung in der zweiten Variante,

Fig. 11

eine vierte alternative Ausführungsform einer erfindungsgemäßen Vorrichtung in der zweiten Variante,

Fig. 12

eine weitere alternative Ausführungsform einer erfindungsgemäßen Vorrichtung in einer dritten Variante,

Fig. 13

weitere alternative Ausführungsformen einer erfindungsgemäßen Vorrichtung in der dritten Variante.

Show it:

Fig. 1

a representation of the overall system when the stator is connected to the electronic potential,

Fig. 2

a modeling of the capacity network,

Fig. 3

an equivalent circuit diagram of the parasitic storage capacities and the main current path of the storage current,

Fig. 4

an equivalent circuit diagram of the parasitic storage capacities when the stator is connected to the earth current path which is diverted as a result,

Fig. 5

a schematically simplified equivalent circuit diagram for determining the bearing voltage according to the embodiment Figure 3 ,

Fig. 6

comparative measurement curves of common mode voltage and bearing voltage (without and with connection of the stator),

Fig. 7

an alternative embodiment of a device according to the invention,

Fig. 8

another alternative embodiment of a device according to the invention in a second variant,

Fig. 9

a second alternative embodiment of a device according to the invention in the second variant,

Fig. 10

a third alternative embodiment of a device according to the invention in the second variant,

Fig. 11

a fourth alternative embodiment of a device according to the invention in the second variant,

Fig. 12

a further alternative embodiment of a device according to the invention in a third variant,

Fig. 13

further alternative embodiments of a device according to the invention in the third variant.

The invention is based on preferred embodiments with reference to the Figures 1 to 13 described, the same reference numerals indicating the same functional and / or structural features.

In the Figure 1 A representation of an exemplary embodiment of an overall system is shown, in which a device 1 for reducing and / or avoiding harmful bearing currents in the three-phase-fed EC motor M is shown. The motor M is designed with a rotor 2 and with an insulated stator 3, which are only shown schematically here.

The device 1 comprises (as further in FIG Figure 1 can be seen) a connection electronics 10 for connecting the motor M with a rectifier 14, an intermediate circuit 15 and an inverter 16 and on the input side with an EMC filter 17. As also in the Figure 1 can be seen, the stator 3 is connected via a connection 20 to the potential tap 20a on the inverter 16.

To explain the mode of operation of the device according to the invention, the Figure 2 a modeling of the 3-phase inverter 16 and the capacitive network of the parasitic capacitances are shown.

The DC link voltage DC +, DC- of the DC link 15 (as in FIG Figure 1 shown) on the inverter 16. The voltages applied to the motor terminals are marked with u _u , u _v and u _w , while the half bridges are shown with the simplified half bridge symbol.

The voltage applied to the motor terminals by the inverter 16 is broken down into a common-mode component (u _CM ) and a push-pull component (u _DM ). The common mode component is made up of the mean value of the voltage provided by the half bridges of the inverter. The push-pull voltage from the difference between the voltage at the switch and the common-mode voltage.

In the Figure 3 there is an equivalent circuit diagram of the parasitic bearing capacities with the main current path of the bearing current through the bearings of the motor M. The rotor 2 is rotatably supported relative to the stator 3 via a bearing outer ring 4a (stator side) and a bearing inner ring 4b (rotor side).

In a simplified, descriptive model, at least the following capacities appear in the system, which in the Figure 3 are specified. Due to the small distance between the winding and the stator package, a winding-stator capacitance C _WS occurs in the system. The winding heads of the motor also create a winding-rotor capacitance C _WR .

The rotor-stator capacitance C _RS , which is comparable to the capacitance of a cylindrical capacitor, is located between the rotor and the stator. Parasitic capacitances occur between the stator 3 and the earth, as well as the rotor 2 and the earth. The capacitive coupling of the iron yoke to the housing cover or the electronics leads to the parasitic stator earth capacitance C _SE ,
while the parasitic rotor earth capacitance C _{RE is} typically caused by attachments to the rotor (for example metal impellers). The bearing capacity between the bearing outer ring 4a and the bearing inner ring 4b is referred to as C _bearing .

A change in the star point potential of the C _WS / C _WR star connection can only be generated by a change in the common mode voltage, which is provided by the inverter 16.

The push-pull component of the voltage, which is only present between the windings, cannot cause a change in the star point potential due to the symmetrical star connection of the capacitances from the winding to the rotor 2 and from the winding to the stator 3 (C _WS and C _WR ). This means that push-pull voltages are not transferred to the remaining capacities of the network.

A closed current path is also required for the formation of common mode currents over the capacitive network. This is generated by connecting the inverter 16 to the earth potential U _{E of} the earth via the Y capacitors, EMC filters or the grid installation. The common mode currents that form thus flow from the inverter 16 to the capacitive network to earth and from there via the filter components and the network back to the inverter 16.

Typically, the largest share is in Figure 3 currents denoted by i _EDM via the stator 3, via the bearing outer ring or bearing inner ring 4a or 4b to the rotor 2 and then to the earth (shown as a dashed arrow line in Figure 3 ).

Because of the different sizes of the capacitances (C _WS significantly larger than the capacitance C _WR ), the common mode currents therefore flow mainly via the capacitances C _WS and, in the case of such a circuit topology, therefore via the bearing rings 4a, 4b.

The larger the additional attachments on the rotor 2, the lower the rotor potential with respect to earth and the lower the bearing tensions. However, only voltage changes ΔU with respect to earth potential U _{E are} relevant for the harmful bearing voltages.

In the Figure 4 the implementation of the invention is shown on the basis of an equivalent circuit diagram of the parasitic storage capacitances in the connection, ie the connection of the stator 3 via an impedance Z at the potential tap 20a of the inverter 16. The positive DC link potential DC + is present here during operation of the motor M. This ensures that the potential of the stator 3 for high frequencies is kept constant with respect to the earth potential U _E and that the majority of the currents i do not flow back via the bearing, ie the bearing outer ring 4a or bearing inner ring 4b, but directly back to the intermediate circuit potential.

If the impedance of Z for the high-frequency bearing currents can be neglected and the connection 20 between the stator 3 and the potential tap 20a behaves like a short circuit, the potential of the stator 3 can also be at a maximum with the mains frequency and no longer with the frequency of the inverter 16 change. The resulting (simplified) equivalent circuit diagram can be used with Figure 5 specify.

Due to the different size of the capacitances (C _WS significantly larger than the capacitance C _WR ), the impedance of the entire route increases accordingly and the bearing tension is reduced. The parasitic capacitance C _RE , which is now parallel to the bearing, further reduces the bearing voltage, as a result of which a significant reduction in the bearing voltage can be achieved with this device.

By connecting the stator 3 to the positive DC link potential DC +, the proportion of high-frequency currents that flow to earth is significantly reduced. This has the further advantage that the volume of the EMC filter does not increase in the solution according to the invention.

In the Figure 6 there are comparative measurement curves for common mode voltage and bearing voltage. The upper illustration shows the case without connection of the stator 3 and the lower illustration the embodiment according to the invention with the connection 20 between the stator 3 and the positive intermediate circuit potential DC +.

In the figure above, the dashed line shows the pulsed DC voltage clock component U _CM and the solid line shows the bearing voltage U _b . In the area around 50us, voltage breakdowns in the bearing voltage can be clearly seen.

In the lower figure of the Figure 6 it can be seen that in the case of an insulated stator 3, by means of its connection to the electronic potential, the amplitudes of the bearing voltage u _b applied to the bearing have been significantly reduced and breakdowns of the bearing voltage do not occur in the bearing.

Figures 7 to 13 show alternative embodiments of the invention according to two further main variants. The first (previously shown) variant concerned the connection of the stator 3 to a potential tap 20a of the intermediate circuit potential. In Figure 7 A solution is shown in which the connection of the stator 3 was not realized with the positive, but with the negative DC link potential DC-. Since only common-mode processes are decisive for the bearing voltage, possible bearing currents that occur via DC link of the intermediate circuit 14 can be closed via the intermediate circuit capacitor C _DC .

The intermediate circuit capacitor C _{DC has} significantly higher capacitances than the parasitic capacitances of the motor and can be assumed to be a short circuit for simplicity. This creates a direct connection to the electronic ground and the potential of the stator 3 can be kept constant for high frequencies with respect to earth.
A second variant relates to any other suitable connection that is at the potential of the intermediate circuit, such as. B. in the Figure 8 a potential tap 20a is shown on the center tap of the intermediate circuit 15.

A third variant relates to a suitable connection which is at a potential of the connection electronics 10 which deviates from the intermediate circuit potential, such as, for. B. in the two figures of the Figure 9 shown, in which the potential tap 20a takes place before the rectifier 14. If, for example, a Y capacitor is used, a connection can also be made on the AC side of the rectifier 14. Such a configuration would correspond to an alternating connection of the stator 3 to the ground potential and the intermediate circuit potential, depending on the sign of the mains voltage currently applied to the rectifier 14. However, it should be noted here that no common mode chokes or filter elements which increase the impedance may lie between the intermediate circuit 15 and the potential tap 20a, in order not to counteract the technical effect of the present invention. In this context, it should also be taken into account that the duration of the rectifier 14 must be given over the longest possible time interval within the mains half-wave. This can be achieved, for example, by an active PFC circuit. In the case of a passive rectifier 14, on the other hand, it must be ensured that either the rectifier 14 has a sufficiently large diode capacitance or, alternatively, corresponding impedances are connected in parallel with the rectifier, which reduce the rectifier impedance for the high-frequency bearing voltage. A corresponding version is in the Figure 10 shown.

Another aspect that has to be taken into account in the various embodiments is the protection against accidental contact of the motor M and thus the protection of persons against electric shock, since the connection 20 between the Stator 3 and the electronics potential, the insulation distance reduced compared to conventional designs and in a first approximation to the insulation distance between the bearing seat and the rotor 2.

One way of producing the required protection against accidental contact is to connect the stator 3 via impedances with adequate protective insulation. For example, Y capacitors can be used for this. For the HF common mode voltage of the inverter 16, the Y capacitance nevertheless acts like a short circuit and thus like a direct connection. Particularly when the capacitance used in the frequency range of the bearing voltage has a significantly lower impedance than the capacitive network.

In the Figure 13 Further embodiments are shown which show the connection 20 of the stator 3 via an impedance Z.

Claims (10)

1. Apparatus (1) for reducing and/or avoiding harmful bearing currents in an electrical machine (M) such as preferably a three-phase EC motor (M), having a rotor (2) and a stator (3) which is constructed in an insulated manner, wherein at least one outer bearing ring (4a) and one inner bearing ring (4b) are provided between rotor (2) and stator (3), comprising a connecting electronics (10) for connecting the motor (M), characterized in that the stator (3) is connected by means of an electrical connection (20) to a high-frequency electronics potential which differs from and is stable with respect to the earth potential (U_E) at a potential tap (20a) of the connecting electronics (10).
2. Apparatus (1) according to claim 1, characterized in that the electrical connection (20) between the stator (3) and the electronics potential is formed by an intermediate capacitance (21) or via an intermediate impedance.
3. Apparatus (1) according to claim 1, characterized in that the electrical connection (20) between the stator (3) and the electronics potential (U_E) is formed as an electrical short circuit connection.
4. Apparatus (1) according to one of the claims 1 to 3, characterized in that the connecting electronics (10) comprises a rectifier (14), an intermediate circuit (15) and an inverter (16), and the potential tap (20a) in the connecting electronics (10) is provided so that the electronics potential connected to the stator (3) corresponds to a potential of the intermediate circuit (15), preferably to the positive intermediate circuit potential (DC+), to a ground potential of the intermediate circuit (15), to a potential at the center tap (15') of the intermediate circuit (15) or to the negative intermediate circuit potential (DC-).
5. Apparatus (1) according to one of the claims 1 to 3, characterized in that the potential tap (20a) for the stator (3) is a tap immediately before or immediately after the rectifier (14).
6. Apparatus (1) according to one of the claims 1 to 5, characterized in that the connecting electronics (10) moreover comprises an EMC filter (17) on the input side, and the potential tap (20a) for the stator (3) is a tap within the EMC filter (17).
7. Apparatus (1) according to one of the claims 1 to 5, characterized in that the connecting electronics (10) is connected to a low-voltage power supply or switching power supply, and the potential tap (20a) for the stator (3) is provided as a tap at the outputs of the low-voltage power supply or of the switching power supply.
8. Apparatus (1) according to one of the claims 4 to 7, characterized in that the rectifier (14) is an active or passive rectifier.
9. Apparatus (1) according to one of the claims 5 to 8, characterized in that a network which reduces impedance in the frequency range of the bearing voltage is connected in parallel to at least one of the diodes (14a) of the rectifier (14).
10. Electric motor (M), preferably an EC motor, formed with an apparatus (1) according to one of the preceding claims 1 to 9.

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